TA2021B amp and Dc offset issues. - diyAudio
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Old 20th July 2015, 10:02 PM   #1
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Default TA2021B amp and Dc offset issues.

Hi all.

I hope this hasnt been covered somewhere else- if so apologies, but i can only find references to this kit on german sites and my german isnt great

Okay so i bought this TA2012B kit from art0.de and have assembled. See pics. Everything turns on, and one channel works great. The other output measures a DC voltage of 0.6V across it's output. I havent connected this to any speakers yet, fearing what it will do to them.

There is a trimpot that adjusts the dc offset, but the corresponding trimpot, doesnt have any affect on the dc offset except to boost the offset to 13v, which faults out the chip and shuts off the amp.

I have measured the voltages for all points i could find on the schematic and is like this:

on the TA2021b chip
1- 5v
2- 16v
3 - 6.69v
11 - 2.55v
15 - 2.5v
16 - 2.4v
36 - 22v

@j4
out 2 (-) - 6.96v
out 2 (+) - 6.34

@j6
out 1 (-) - 6.67v
out 1 (+) - 6.62v

the common point between rj50k (the two trims) - 5.08V


I have also taken the trimpot off the board and checked that it measures 50k across lugs 1 and 3.

Anyone have any idea what i can do to diagnose and fix this problem. Sorry if this is super noob, ive been building guitar pedals for ages, but this is my first amp.

SCHEMATIC: http://www.art0.de/downloads-ta2021b...uit-300dpi.pdf
DATASHEET: http://www.art0.de/images/ta2021b.pdf

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Thanks heaps in advance!
Steve
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Old 21st July 2015, 02:01 AM   #2
wwenze is offline wwenze  Singapore
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Connect some load and see if it disappears.

0.6V won't kill if it's for a few minutes... or hours..
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Old 21st July 2015, 09:35 AM   #3
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Quote:
Originally Posted by wwenze View Post
Connect some load and see if it disappears.

0.6V won't kill if it's for a few minutes... or hours..

Still 0.59V...
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Old 24th July 2015, 06:40 PM   #4
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No one have any insight into what might be causing it? I am happy to tear apart and replace parts, but im not sure exactly what i should be looking at that would cause it?

Any help would be greatly appreciated!
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Old 24th July 2015, 06:43 PM   #5
Mooly is offline Mooly  United Kingdom
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Was the 0.59 volts with a speaker as a load ? How does the good channel compare ?
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Old 24th July 2015, 06:46 PM   #6
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Quote:
Originally Posted by Mooly View Post
Was the 0.59 volts with a speaker as a load ? How does the good channel compare ?
Hey, yeh that was using the same speaker as on the working channel. The working channel was measuring at 0.1mv

Thanks for the response!
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Old 24th July 2015, 06:50 PM   #7
Mooly is offline Mooly  United Kingdom
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I'm just looking at the circuit. Does the offset pot on the good channel have the ability to trim the offset 'either way' so that while keeping the meter leads the same across the load, the offset can be swung both positive and negative.

How does the bad channel compare in the sort of adjustment range possible ?

Is the 'feel' of the pot similar between channels... not the physical feel but the rate at which the offset adjusts.

There is not much else beside the IC really for this, assuming that all parts are correctly fitted and the correct values.
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Old 24th July 2015, 06:50 PM   #8
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by skadagladje View Post
Hey, yeh that was using the same speaker as on the working channel. The working channel was measuring at 0.1mv

Thanks for the response!
Does the bad channel actually sound OK... thats a big clue.

Back in a little while
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Old 24th July 2015, 06:56 PM   #9
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Quote:
Originally Posted by Mooly View Post
Does the bad channel actually sound OK... thats a big clue.

Back in a little while
Im at work at the moment so i will have to test again and get back to you. I was anxious about killinng expensive speakers with a high dc offset so i didnt really test it sonically too thoroughly...
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Old 24th July 2015, 07:50 PM   #10
Mooly is offline Mooly  United Kingdom
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The offset is way below any point damage would occur, 0.6 volts is only 120 milliwatts even if the speaker were 3 ohm. For a nominal 8 ohm load its just 45 milliwatts.

You really need to observe how the offset pot on the good channel behaves. For example is the good channel pot centred to give zero offset (or is it almost at one end) and is the bad one at one end and still not going low enough. Does the good and bad channel offsets increase by similar amounts 'per degree' of pot rotation or is the bad channel totally different.

Something else which I'll just mention... some DVM's get upset by any high frequency noise that is present on the voltage being measured, and this being a Class D amp is really all filtered noise. It might be worth you adding a series resistor and cap across the load and then measuring the offset by taking the voltage across the cap. That would form a filter and remove much of the noise. See if the readings are different doing it that way. Resistor can be anything over say 10k and the cap something like a 10uf electrolytic.
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